1. Field of the Invention
The invention is concerned with improving operations in cracking of hydrocarbons in the absence of added hydrogen. Characteristically, commercial equipment for this purpose involves a cracking reactor and a regenerator with continuous circulation of catalyst through the two vessels. Particularly significant advantages are achieved in Fluid Catalytic Cracking (FCC) with zeolitic catalyst. The invention has no applicability to hydrocracking using a fixed bed of catalyst and an excess of added hydrogen.
2. Discussion of the Prior Art
Two general types of catalytic cracking process are currently in commercial use. Thermofor Catalytic Cracking (TCC) uses a moving compact bed of catalyst in both reactor and regenerator. Catalyst which has become relatively inactive by deposition of a carbonaceous deposit commonly called "coke" is continuously withdrawn from the bottom of the reactor in which gas oil is cracked by contact with the catalyst at elevated temperature. The spent catalyst from the reactor is passed to the top of a regenerator in which activity of catalyst is restored by burning the coke in air. Hot regenerated catalyst from the bottom of the regenerator is continuously returned to the top of the reactor to repeat the cycle first described.
In Fluid operations, the catalyst follows a similar circulation but is "fluidized" in both reactor and regenerator by upwardly flowing gases in each, hydrocarbon vapor in the reactor and in the regenerator.
The major difference in catalyst for the two processes is particle size. Pellets or beads of about 1/8" diameter are employed in TCC. Fine powders, with an average particle size of about 70 microns, are used in FCC.
Both types of process are operated at pressures from atmospheric to about 40 psig. Hydrogen is produced in small amounts by the cracking reaction, but no hydrogen is added as such to the reactors of TCC and FCC Units. This cracking in the absence of added hydrogen is endothermic, as contrasted with the exothermic character of hydrocracking with a catalyst which contains significant amounts of hydrogenation metal (upwards of 0.5 wt. %) and in the presence of large excess of added hydrogen, as described in U.S. Pat. No. 3,173,854.
In general, potent hydrogenation metals are avoided in TCC and FCC catalysts. A serious problem for these catalysts is recognized in cracking of stocks which contain metals. Particularly disadvantageous is deposition of the Group VIII metal nickel. Amounts of nickel on the order of 0.03 wt. % on catalyst will increase the hydrogen make to a level which causes severe problems in handling of the dry gas from TCC and FCC operations.
The catalysts employed in FCC and TCC have included acid treated clays, amorphous silica-alumina composites and the like. Many variants, such as silica-zirconia, silica-magnesia and other acidic porous solids have been described in the literature.
More recently, very effective catalysts have been prepared by blending a major portion of the older amorphous catalysts with a minor portion of an active crystalline aluminosilicate zeolite. Typical catalysts of this type for FCC and TCC are described in U.S. Pat. Nos. 3,140,249 and 3,140,253, the disclosures of which are hereby incorporated by reference.
In FCC and TCC a problem arises from incomplete combustion, leaving a significant amount of carbon monoxide (CO) in the flue gas. Aside from the undesirability of discharge of CO to the atmosphere, such flue gases tend to burn (by reaction of CO with residual oxygen in the flue gas) in ducts and flues of the plant, damaging these structures by excessive temperatures.
It has been proposed to alleviate the CO problem in TCC by adding a small amount of chromic oxide to the catalyst. This causes some impairment of gasoline yield, but that effect is tolerable in combatting the CO problem. See U.S. Pat. No. 2,647,860.